Optical fiber refractive index gratings are widely used in fiber optical communication systems. In particular, fiber Bragg gratings are used as spectral filters for wavelength division multiplexing (WDM) of optical signals. In the WDM systems, it is important that the central wavelength of the filter does not change. However, as the temperature of the fiber Bragg grating rises, the central wavelength fluctuates, typically by 10 pm/.degree. C. Such wavelength variation has a detrimental effect on the performance of the WDM systems.
It is well known that the central wavelength of a fiber Bragg grating varies with temperature by the following expression: ##EQU1##
.lambda..sub.0 is the wavelength at a grating's temperature T.sub.0, and K.sub.T is the thermal expansion coefficient of the fiber Bragg grating. The thermal expansion coefficient of a silica-based fiber Bragg grating has a typical value around 6.about.7.times.10.sup.-6 /.degree. C.
In the prior art, there exist several mechanisms to produce temperature insensitivity to the central wavelength of the fiber Bragg grating. The actual amount of compensation required for a particular grating depends on the composition and structure of the grating. One way is to adopt an active system, which depends on a feedback from a temperature active element such as a thermal electric cooler. Another way is to use a passive method. The passive method is more desirable because it does not require power consumption and control logics to maintain a constant wavelength. In one approach, wavelength control under the passive method is accomplished by clamping the fiber containing the fiber Bragg grating with a mechanical structure made of multiple materials with different thermal expansion properties, usually positive thermal expansion coefficients. U.S. Pat. No. 5,042,898 and PCT/US97/23415 provide examples of the multiple materials assembled to produce a compressive strain as the temperature of the package increases. However, these mechanical structures are complicated to fabricate and expensive.
Another approach is to use isotropic materials that have negative thermal expansion coefficients that precisely match the wavelength shift of the fiber Bragg grating. Examples of this approach were presented at the 22.sup.nd European Conference on Optical Communication (ECOC'96) in Oslo, by D. L. Weidman, et al. of Coming Inc. and U.S. Pat. No. 5,694,503. The composition of a glass ceramic is designed to have a thermal expansion coefficient which matches the fiber Bragg grating's thermal expansion coefficient. However, the precise control of the thermal expansion of a glass ceramic to a desire level is difficult to achieve. Moreover, the design and fabrication of these exotic materials increase expense and complexity in the production these structures.